1. Technical Field
The invention relates to photosensitive materials and processes for making same and is especially, but not exclusively, applicable to photosensitive materials suitable for image-wise recording, for example holographic recording of data or volume holograms.
2. Background Art
There are many substances that are known to be photosensitive and suitable for holographic image recording. Those include silver halides, photopolymers, dichromated gelatins, and photorefractive, ferromagnetic, photochromic, photodichroic and azo-dye materials. In these materials, temporary or permanent changes of real or imaginary part of refractive index are induced upon-illumination of the material by actinic radiation and, typically, subsequent processing steps are required.
For high-resolution phase imaging such as that required in practical holographic data storage devices, there are several properties that an optimum recording material should exhibit. These include high dynamic range and sensitivity, high resolution, negligible shrinkage upon holographic exposure, low noise, phase image forming mechanism, image and dimensional stability, and fabricability into thick films and monoliths, to name the most relevant ones. However, to meet all these properties in a single material is a significant challenge that has not been satisfactorily overcome up to present and lack of such medium is currently considered in the art as one of the main hurdles in development of practical holographic data storage systems. A comprehensive review of holographic recording materials for data storage applications can be found in Holographic recording materials for optical data storage, by P. Cheben, in Advanced Optics, Chapter 1, pp. 565–609 (Ariel, Barcelona, 2002).
Existing Techniques
Photopolymers are believed to be promising holographic recording materials for applications such as display or security holograms, Write-Once Read-Many times (WORM) holographic memories, or Holographic Optical Elements (HOEs). A number of photopolymerizable systems have been developed up to present, such as those disclosed in U.S. Pat. Nos. 3,658,526; 4,942,112; 4,959,284 and 4,994,347 assigned to E. I. Du Pont de Nemours and Company, U.S. Pat. Nos. 4,588,664; 4,696,876 and 4,970,129 assigned to Polaroid Corporation, U.S. Pat. No. 3,993,485 assigned to Bell Telephone Laboratories, U.S. Pat. No. 4,173,474 assigned to Cannon Kabushiki Kaisha, and U.S. Pat. No. 3,694,218 assigned to Hughes Aircraft Company. Another promising class of materials are porous glasses impregnated by photopolymerizable materials such as those disclosed in Spanish patent P9700217 assigned to the Instituto Nacional de Tecnica Aeroespacial, Japanese patents JP6148880 assigned to Nippon Sheet Glass Co. Ltd. and JP61141476A assigned to Sony Corp.
Limitations or Drawbacks of Existing Techniques
Application of photopolymers to volume holography and data storage is severely limited due to their limited thickness, high shrinkage during holographic exposure and need for solvent processing. A recording medium of millimeter thickness or more and exhibiting high photoinduced refractive index change is required to achieve high storage density by recording multiple volume holograms, separated from each other by the Bragg effect, in the same spatial location.
The most important limitations are imposed by organic polymeric binders that limit thickness of the medium, usually to less than a few hundred microns, and temperature- and light-induced dimensional changes that can distort the holograms and degrade the fidelity with which the stored images can be retrieved. An approach to prepare thicker photopolymers is to use resins consisting of two independent photopolymerisable systems such as those disclosed in L. Dhar et al., Opt. Lett. 24, pp. 487–489 (1999), , in which matrix-forming oligomers are first precured to a gel state and then the hologram is recorded by photoinduced polymerization of monomers dissolved in the resin. However, this approach does not simultaneously achieve high dimensional stability and maximum photoinduced refractive index change. To increase the rigidity of the material, higher levels of precuring are required, which decreases diffusional mobility of the monomer in the resin and degrades holographic properties of the photopolymer. In addition, some monomeric species are inevitably consumed (polymerized) during the precuring step, which in turn reduces dynamic range of the holographic recording and, ultimately, limits the data storage capacity of the material.
Porous glasses such as those fabricated by sol-gel technique appear to be promising for data storage as they can typically be fabricated as bulk monoliths. However, the materials disclosed up to present, including Spanish patent P9700217 assigned to the Instituto Nacional de Tecnica Aeroespacial, Japanese patents JP6148880A assigned to Nippon Sheet Glass Co. Ltd. and JP61141476A assigned to Sony Corp. suffer from drawbacks ranging from insufficient sensitivity and dynamic range for practical applications to insufficient optical quality and thickness.